Process of controlling paint baths in electrodeposition

ABSTRACT

THE COMPOSITION OF AQUEOUS PAINT BATHS USED IN ELECTRODEPOSITION OF PAINTS IS CONTROLLED BY MEASURING THE ZETA POTENTIAL. A ZETA POTENTIAL OF AT LEAST -40 MILLIVOLTS IS MAINTAINED.

3,764,505 PROCESS OF CONTROLLING PAINT BATHS IN ELECTRODEPOSITION Joseph M. De Vittorio, Homewood, Ill., assignor to The Sherwin-Williams Company, Cleveland, Ohio No Drawing. Continuation of abandoned application Ser. No. 618,342, Feb. 24, 1967. This application Apr. 15, 1971, Ser. No. 134,496

Int. Cl. B01k 5/02; C23b 13/00 US. Cl. 204-181 8 Claims ABSTRACT OF THE DISCLOSURE The composition of aqueous paint baths used in electrodeposition of paints is controlled by measuring the zeta potential. A zeta potential of at least 40 millivolts is maintained. q

This application is a continuation of copending US. application Ser, No. 618,342 filedjFeb. 24, 1967, now an The expression aqueou s p t bath as used herein refers to a system inf'w disc tejparticles of a paint coating composition are'suspencle I water.

' found that there are a numle rodepositing paints from aquei r e'd atnountof paint can be applie d qulite a'c'our ely, even Onjcontpnred surfaces such as, for enamplefl'a oniobile bbdies',tra mes, wheels, and other objects. Uniforrn coverage'i's" obtained without bare spots.

, One 'problein'has been the difliculty in controlling the composition of the paint bath. Aselectrodeposition proceeds, the solids in the bathare electrodeposited thereby changing the stabilityqfthe suspension. Where the bath becomes unstable; uniform coatings are no longer obtainedl'ltjwouldbe "an advantage therefore, to be able to determine in advancewhen thefpal t bath is likely to become unStabIe during electro'd'epo sition and to control the composition 1 of"the enaisoas' toavoid such instability. In accordance with'thef'inv'ention, i t has been found that in the electrodepos'itio of paint trom, an aqueous paint bath thezeta potential changes as the electrodeposition proceeds and e'ventuallyreaches' apoint at which the bath is unstable and is, no longer effectively useful. By measuring the zeta potential of; the bath perodically and adjusting the composition, of the bath before this point is reached so as to m aint ia predetermined zeta potential, the electrodeposition' can be continfied'for a long period of time .without discarding the.bath. Thus, in the electrodeposition of paint onto ,an anodicobject from an aqueous paintbath in'which th'e particles suspended in water are electronegative, as 'is nonnallyfihe case, it has been found that'the compositionof the' bath should be periodically adjusted to maintainja'zetalpotential above about -40 millivolt's'. (mv.").,In'most' cases the bath becomes completely unstable when th'e'j z'etapotential is as low as In practicing'the invention, the'general procedure is to measure the zeta potential ofa given paint bath periodical- 1y while electrodepositing paint therefrom until the'point of instability is"-'reache"d; .The Zeta-potential atthis point then becomes .a point of reference "for future operations in electrodepositingpaint from: thesameior'similar paint baths, and .suc'h1operations are conducted .whileadding or removingfrom the. paint-bath, usually periodically, whatever ingredientsare required, to maintain the bath at a predetermined.-; zeta potential jWheieIhfi iaqueous paint suspensionis stable.- The zeta potentialis easuredby meansof a Zeta- Meter. The Zeta-Meter. is a known instrument consisting of a continuously variable .0-500, volts direct current power'supplywith a fidelity (reversing switch and a pre- 3,764,505 Patented Oct. 9, 1973 cision volt meter and Inicro-arnmeter. The unit is also equipped with a variable voltage outlet for a standard microscope illuminator, and an electric timer which measures to tenths of seconds. The microscope is stereoscopic and built with a special mechanical stage and ocular micrometer. A heat-absorbing cell holder with a mirrored back acts to reflect a beam of light through the cell tube at an angle such that the direct light is removed from the Optics of the microscope. The electrical leads from the power source are attached to the electrodes of an electrophoresis cell. The electrophoresis cell has two iridiumhardened platinum electrodes. One is a strip type and the other a tube.

In operation, a sample of the material whose zeta potential is to be determined is highly diluted, for example, to the extent of about five drops to 250 ml. of water. The samples are mixed for 10.5 minutes on a shaker, then placed in the cell unit, the unit is energized, preferably with a current of volts D.C., and the particles are counted and timed as they pass over the ocular micrometer scale. The time for passage of 100 particles is recorded and an average time is determined.

When a sample of material containing charged particles is placed in an electric field, the anions migrate to the positive electrode and the cations to the negative electrode. This attraction for the particles increases as the charge on the particles in question is increased. The velocity of a colloidal particle in an electric field is directly proportional to that field and to the zeta potential of the particle.

The Helmholtz-Smoluchowski formula is the classical mathematical representation of zeta potential.

where Em=electrophoretic mobility in microns/sec. per volt/cm., Vt=the viscosity in poises at any given temperature, and Dt=the dielectric constant of the suspending media at any given temperature. This formula as expressed has been demonstrated to be primarily applicable to large, non-conducting particles. If small non-conducting particles are considered, then the Hiickel equation for spherical particles is used.

Vt ZP Em-Gn- Di It is readily seen that this equation dilfers from the first only with respect to the use of 61r in place of 41r. The value of 61r is more applicable to spheres, whereas this value for elongated (oblate) particles varies from 412' to 811- depending on particle orientation with respect to its direction of migration.

Henrys formula combined the aspects of both the Helmholtz-Smoluchowski formula and the Hiickel formulae in the following expression:

the factor f(Ka), the Henry factor, is a function of the product of the particle radius (a) and the Debye-Hiickel constant, K.

Because the smallest particle visible with the optical microscope used with anele ctrophoresis cell is approximately 1000 to 2000 A. (0.1 to 0.2 microns) the Helmholtz-.Smoluchowski and Henry formulae are generally used.

If the particles of the material to be tested'are negatively charged, they move from the cathode to the anode and if they are positively charged, they move from the anode to the cathode. Due to the fact that the sample is highly diluted, the viscosity and the dielectric constant are practically the same as water and can be discounted in computing the zeta potential. Hence, the zeta potential is directly proportional to the average speed of the particle.

In preparing the sample it is desirable to use a good grade of distilled water which has been filtered so that all particulate material down to 50 millimicrons in size has been removed.

The invention will be further illustrated but is not limited by the following examples in which the quantities are stated in parts by weight unless otherwise indicated.

EXAMPLE I (A) 212 parts medium chrome yellow, 8.4 parts molybdate orange and 431 parts of a water soluble acrylic resin were ground on a roller mill to a Hegman fineness of 7H.

(B) The composition from A was mixed with 218 parts water soluble acrylic resin, 117.2 parts hexamethoxymethylmelamine, 44.8 parts butylether of ethylene glycol, 7.2 parts xylene and 4710.6 parts of deionized water. The resultant composition contains about 12% by weight nonvolatile solids.

(C) The water soluble acrylic resin used in A and B was prepared by heating together 56 parts of butylacrylate, 23 parts of styrene and 14 parts methacrylic acid at 300 F. for 3-3 /z hours with the addition also of 10 grams per gallon of cumene hydroperoxide and 20 grams per gallon of azobisisobutyronitrile. One-half hour before the end of the cook, 7 parts of dimethylethanolamine was added. The resultant product was then diluted with 100 parts of a solution containing 48 parts of water and 52 parts of the butylether of ethylene glycol.

(D) The paint prepared as previously described was electrodeposited at 90-100 volts direct current at a temperature of 84-87 F. and a pH of 8.5-8.7 in a one gallon cell. The resistance was 1000-1200 ohms. The anodes were 2%" x Q-steel panels. In each case a coating of approximately 0.9-1.0 mil thickness was deposited in a period of 90 seconds and the coated anode was baked at 350 F. for 20 minutes. The electrodeposition was carried on twice daily for 24 days. During each day the zeta potential was determined in a manner previously described by using a Zeta-Meter and dispersing 5 drops from the bath in 250 ml. of filtered deionized water. In each determination 100 observations were made and the median time of the particles was noted. From this the zeta potential was calculated and it was that the zeta potential was between 50 and 55 mv. during the first ten days and then began to drop oif until it was -41 mv. on the eighteenth day. On the twentieth day the zeta potential dropped to --39 mv., it was slightly lower on the twentyfirst day and on the twenty-fourth day dropped to --36 mv. During the last 4 days there was evidence of nonuniformity in the deposition and instability in the paint suspension. The median time of the particle count varied from an average of 3.85 seconds at the beginning of the tests to 6 seconds at the end.

This test demonstrated that the paint bath lost its stability as the zeta potential dropped and a minimum practical point where the bath was still stable, but was approaching instability, was at a zeta potential around 40 mv.

EXAMPLE II (A) A paste was prepared by mixing together 212 parts medium chrome yellow, 8.4 parts molybdate orange, 454.5

parts water soluble polyester resin, and 5.5 parts triethanolamine and grinding the mixture on a roller mill to a Hegman fineness of 7H.

(B) The product from A was mixed with 299.5 parts water soluble polyester resin, 3.6 parts triethanolamine, 97.4 parts Cymel 301 and 4792.4 parts deionized water.

(C) The polyester was prepared by reacting together 4.48 parts trimethylolethane, 40.37 parts neopentyl alcohol, 27.29 parts azelaic acid and 28.86 parts trimellitic anyhdride. This was dissolved in a solvent consisting of 93.56% water and 6.44% dimethylethanolamine in proportions such that the ratio of amine to carboxy was .75.

The solids content was about 43.85% by volume and the viscosity 1238 poise at 20 C.

(D) The paint bath prepared as above described had a resistance of 550-560 ohms and a pH of 6.9-7.1. It was placed in a gallon cell and electrodeposited at 100-110 volts direct current for seconds at 84-87 F. on anodes consisting of 2% x 5%" Q-steel panels. The coated panels were baked for 20 minutes at 350 F. The film thickness was approximately 0.9-1.0 mil.

Two panels were electrodeposited each day, one in the morning and one in the afternoon. The zeta potential of the bath was determined daily in the manner previously described using a sample consisting of 5 drops of the .bath diluted to 250 ml. with filtered deionized water. The median time in seconds for observations was determined and the zeta potential was calculated.

The zeta potential was initially -64 mv. At the end of 3 days it had dropped to 5 1 mv. 50 parts of the composition from A was then added to the paint bath. After 2 more days the zeta potential dropped to -48 mv. and 20 parts more of the composition from A was added. The zeta potential then changed to -55 mv. The bath was allowed to remain for 3 days without electrodeposition. The zeta potential was then determined to be 54 mv. On the following day the zeta potential had dropped to 50 mv. and 20 parts more of the composition from A was added. The zeta potential then rose to -55 mv. On the next day it dropped to -52 mv. and 20 parts more of the composition from A was added. The zeta potential was restored to about -55 mv. Thereafter, over a period of 14 days, no additions were made to the bath and the zeta potential gradually dropped to -41 mv. At this point it appeared that the bath was approaching instability.

Test have also been carried out with various types of resin solutions without pigments and resins have been deposited from such solutions. In every case it has been found that the zeta potential is an important factor in determining the capability of a given electrodeposition bath. It has also been found that the best results are ob tained at relatively higher zeta potentials and particularly at zeta potentials above 40 mv. In one series of tests an acrylic resin solution was determined to have a zeta potential of -76.5 mv. The same resin solution mixed with phthalocyanine blue pigment at a pigment volume concentration of 3% had a zeta potential of 50.7 mv. When the pigment used was indanthrene blue the zeta potential was 47.2 mv. When the pigment used was titanium dioxide the zeta potential was -59.6 mv.

Since the zeta potential is a measure of the mobility of the particles being electrodeposited, it seems evident that better deposition is obtained when the particles have relatively high mobility. As the more mobile particles are electrodeposited, it is therefore desirable to replenish these particles and thereby maintain a relatively high zeta potential.

The invention is generally applicable to controlling the electrodeposition of coatings from an aqueous coating composition electrodeposition bath containing particles capable of being electrodeposited. The electrodeposition of various types of resins with or without pigments is well known in the art. A number of proposals have been made involving the use of acidic resins. For example, any of the resin-containing paints disclosed in U.S. Pat. 3,23 0,162 can be employed in the practice of the invention.

In general, especially satisfactory results have been obtained where the aqueous coating composition contains 2% to 15% by weight solids.

The invention has been found to be very useful in the electrodeposition of acrylic resins as illustrated in Example I and in the electrodeposition of polycarboxylic-polyhydric alcohol polymeric polyesters as illustrated in Example H. Both types of resins are preferably electrodeposited in a fusible state, together with a cross linking agent such as, for example, hexamethoxymethylmelamine or a fusible melamine-formaldehyde resin or a fusible ureaformaldchyde resin. After the coating has been electrodeposited, it is baked usually at a temperature within the range of SOD-450 F. and this converts the fusible resins to an infusiblestate in which they are very insoluble in water and form excellent protective coatings. The coating compositions can contain various types of water solubilizing agents, such as, for example, amines and alkali metal hydroxides.

The ingredients normally used in preparing the resins to be electrodeposited are well known and require no detailed elaboration. The acrylic resins are formed by polymerization through the olefinic double bond. The polycarboxylic-polyhydric alcohol polymeric, polyesters are formed by polymerization which produces recurring polycarboxylic and polyhydric alcohol residues. The resultant product has a linear structure if dicarboxylic acids are reacted with diols. If polycarboxylic acids or polyols having a functionality greater than 2 are employed, cross linking occurs. It is desirable to control the cross linking in the initial polyester so that it is substantially soluble in water.

I claim: 7

1. In aprocessof controlling the electrodeposition of coatings from an aqueous coating composition electrodeposition bath containing particles capable of being electrodeposited and wherein the coverage becomes non-uniform as saidbathis depleted of said particles, the steps which comprise determining the average zeta potentials of said bath particles periodically during electrodeposition until a point in the electrodeposition is reached wherein uniform coverage is no longer obtained, thereby establishing a range of average zeta potentials at which uniform electrodeposition coverage is obtained and a minimum average zeta potential below which a uniform coverage is not obtained, and thereafter conducting said electrodeposition while periodically determining the average zeta potentials of the bath particles and periodically adding to said bath additional quantities of the ingredients previously removed from the bath by electrodeposition in the proportions in which they were coated out, said quantities being suflicientto maintain said average zeta potentials of the bath particles within said range and above said minimum.

2. A process as claimed in claim 1 in which the particles to be electrodeposited are electronegative and the proportions are controlled to maintain an average zeta potential of at least -40 millivolts.

3. A process as claimed in claim 1 in which said particles comprise at least one resin.

4. A process as claimed in claim 1 in which said particles comprise at least one resin and at least one pigment.

5. In a process of controlling the electrodeposition of coatings from an aqeuous paint electrodeposition bath containing electronegatively charged resin particles and pigment particles capable of being electrodeposited and wherein the coverage becomes non-uniform as said bath is depleted of said particles, the steps which comprise periodically measuring the zeta potential of said bath and periodically adding to said bath additional quantities of said resin particles and pigment particles in the proportions in which they were coated out in sufficient amount to maintain an average zeta potential of the bath particles of at least 40 millivolts.

6. A process as claimed in claim 5 in which said electrodeposition bath contains as essential components a pigment and an acidic resin from the group consisting of acrylic resins and polycarboxylic polyhydric alcohol polymeric polyester resins and an aqueous concentrate comprising said pigment and said resin is periodically added to said bath during said electrodeposition in an amount sutficient to maintain an average zeta potential of the bath particles of at least -40 millivolts.

7. A process as claimed in claim 5 in which said resin is an acidic resin.

8. A process as claimed in claim 5 in which said resin is an acrylic resin.

References Cited UNITED STATES PATENTS 2,894,888 7/1959 Shyne 20418l 3,304,250 2/1967 Gilchrist 204-181 3,575,090 4/1971 Gilchrist 204-181 HOWARD S. WILLIAMS, Primary Examiner UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. I 3,764,505 DATED 1 October 9 1973 INVENTOR(S) 1 Joseph M. DeVittorio It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 45, after "was", second occurrence, insert --found--.

Column 6, line 40, "3,575,090" should read --3,575,909--.

Signed and sealed this 6th day of May 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH c. MASON Commissioner of Patents Attesting Officer and Trademarks 

